Study on tribological properties of multi -walled carbonnanotubes/epoxy resin nanocompositesB.Dong,Z.Yang,Y.Huang and H.-L.Li*College of Chemistry and Chemical Engineering,Lanzhou University,Lanzhou 730000,PR ChinaReceived 16September 2005;accepted 19September 2005Multi-walled carbon nanotubes/epoxy resin (MWNTs/EP)nanocomposites with different MWNTs contents have been prepared successfully.The influence of MWNTs on the friction and wear behaviors of the nanocomposites was investigated by a friction and wear tester under dry-sliding contact conditions.The relative humidity of the air was about 50±10%.Contrast to pure EP,MWNTs/EP nanocomposites showed not only higher wear resistance but also smaller friction coefficient.MWNTs could dramatically reduce the friction and improve the wear resistance behaviors of the nanocomposites.The mechanisms of the significant improvements on the tribological properties of the MWNTs/EP nanocomposites were also discussed.KEY WORDS:epoxy resin,multi-walled carbon nanotubes,nanocomposites,tribological properties1.IntroductionEpoxy resin is well established as thermosetting matrices of advanced composites,displaying a series of interesting characteristics,which can be adjusted within broad boundaries [1–5].They are used as highgrade synthetic resins,for example,in the electronics,aero-nautics,and astronautics industries.Now EP is widely used in architecture,automotive,air and railway trans-port systems for tribological applications.However,the most difficulty in the tribological applications is rela-tively poorer wear resistance for EP.Along with the extensive applications of EP for tribological purposes,the understanding the tribology of EP is becoming increasingly important.There have been several studies on the mechanical properties of CNTs/EP nanocom-posites [6–8].In terms of mechanical properties,CNTs are quite stiffand exceptionally strong,meaning that they have a high Young’s modulus and high tensile strength.Measured Young’s moduli of MWNTs are as high as 1.8TPa and bending strengths as high as 14.2GPa [9,10].These articles have stated that the mechanical behaviors of CNTs/EP were significantly increased by the incorporation of CNTs.Due to the effects of the reinforcement,CNTs can be used to fab-ricate the nanocomposites with excellent tribological properties.The tribological behaviors of many CNTs-based nanocomposites [11–17]have been investigated.It was found that CNTs-based nanocomposites exhibited lower friction coefficient and wear rate compared withthe pure substrates matrix,which resulted in the improvements on reduced friction and wear resistance.Because MWNTs have superior mechanical proper-ties and a high aspect ratio,they are potential excellent reinforcing agent.So MWNTs/EP nanocomposites would be expected to significantly improve the tribo-logical properties of EP-based nanocomposites.How-ever,few reports have been available on the tribological behaviors of MWNTs/EP nanocomposites.Considering the above factors,MWNTs/EP nano-composites with excellent tribological properties have been proposed and studied.To the best of our knowl-edge,this paper firstly deals with the tribological behavior of MWNTs/EP nanocomposites.In this paper,MWNTs/EP nanocomposites were prepared success-fully.The influence of MWNTs reinforcing on the tri-bological properties of the nanocomposites were investigated.The improved friction and wear mecha-nisms of the nanocomposites in dry sliding against a plain carbon steel counterpart were also discussed.2.ExperimentalThe MWNTs used in this work were synthesized by a catalytic chemical vapor deposition (CVD)method [18].After subsequent purification treatment,the as-synthe-sized MWNTs were characterized by a transmission electron microscope (TEM,JEM-1200EX,Japan).EP and MWNTs were thoroughly mixed in acetone by ultrasonic and mechanical stirring.After the acetone had evaporated,30Â7Â6mm block specimens were prepared by compression molding at a pressure of*To whom correspondence should be addressed.E-mail:lihl@1023-8883/05/1200–0251/0Ó2005Springer Science +Business Media,Inc.Tribology Letters,Vol.20,Nos.3–4,December 2005(Ó2005)251DOI:10.1007/s11249-005-8637-850MPa.Then the specimens were sintered at403K, held for3h,and cooled to ambient temperature.The weight content of MWNTs in the nanocomposites ranged from0to4.0wt.%.Finally the resultant speci-mens were prepared for friction and wear tests.The structure of MWNTs/EP nanocomposites was charac-terized byfield emission scanning electron microscopy (FE-SEM JSM-6335F-NT).The microhardness measurements of specimens were performed using a microhardness indenter(VDMH-5 Version2.01,China).A load of10gf with a loading time of5s was used.Each specimen was measuredfive times,respectively.The average result of measurements was taken as the reported hardness value in this paper.The friction and wear tests for evaluation of the tri-bological properties of MWNTs/EP nanocomposites were conducted on an M-2000model friction and wear tester under dry-sliding contact conditions.The plain carbon steel ring(hardness of HRC48–50)in a diameter of40mm was used as the counterpart.Sliding was performed at sliding velocity of0.431m/s,normal load of50N,and test duration of1h.The ambient tem-perature was roughly25°C and the relative humidity was about50±10%.Before each test,the surfaces of the block specimen and the counterpart ring were abraded with No.900water-abrasive paper.Then the steel ring and the block specimen were cleaned with acetone.The resultant surface roughness of both the specimen and the stainless steel ring ranged from0.2to0.52l m.The friction coefficient of each specimen was calculated by taking into account the normal load applied and the friction force measured.The friction coefficient was re-corded under steady-state conditions by a personal computer,which controlled the friction and wear tester. At the end of each test,the width of the wear scar on the block specimen was measured with a digital optical microscope with an accuracy of0.01mm,and then the wear rate(x)of the block specimen was calculated.The average result of the three replicate tests was reported in this paper.The deviation of the data of the replicate friction and wear test was10%.3.Results and discussionFigure1shows TEM image of MWNTs.After being purified well,MWNTs are central hollow tubes and have large aspect ratio.The outer diameters of most MWNTs range from10to20nm,and their lengths are several micrometers.Figure2shows FE-SEM image of fractured surface of the MWNTs/EP nanocomposites.The image showed that MWNTs dispersed well in the EP matrix and con-tacted with EP closely.Figure3gives the microhardness of the MWNTs/EP nanocomposites as a function of MWNTs content.The microhardness of the nanocomposites increases sharply when the MWNTs content is below1.5wt.%.The mi-crohardness values decrease slightly when the MWNTs content is above 1.5wt.%.It is attributed to the conglomeration of MWNTs in thenanocomposites Figure1.Typical TEM image ofMWNTs.Figure2.Typical FE-SEM image ofMWNTs/EP.Figure3.Variation of the microhardness of MWNTs/EP nanocom-posites with MWNTs content.252 B.Dong et al./Multi-walled carbon nanotubes/epoxy resin nanocompositesmatrix.It is thus inferred that the incorporation of MWNTs as a reinforcing agent helps to increase the load-carrying capacity and mechanical properties of EP.Figure4shows the friction coefficient of MWNTs/EP nanocomposites as a function of MWNTs content for steady-state sliding against the stainless steel ring under dry-sliding contact conditions.It is apparent that the friction coefficient of MWNTs/EP nanocomposites de-creases with increasing MWNTs content.The friction coefficient values of nanocomposites sharply decrease when MWNTs content is below1.5wt.%.As the con-tent of MWNTs in nanocomposites is higher,the fric-tion coefficient becomes lower and keeps a relatively stable value.Figure5indicates the effects of MWNTs content on wear rate of MWNTs/EP nanocomposites.It can be clearly seen that the incorporation of MWNTs signifi-cantly decreases the wear rate of EP.The wear rate of MWNTs/EP nanocomposites decreases sharply from 2.7Â10)5to6.0Â10)6mm3N)1m)1with the concen-tration of MWNTs from0to1.5wt.%.It is found that 1.5wt.%MWNTs/EP nanocomposites exhibit the smallest wear rate.When the content of MWNTs in the nanocomposites exceeds 1.5wt.%,the wear rate of MWNTs/EP nanocomposites increases slightly with increasing MWNTs content.Similar results were also observed on the microhardness of the nanocomposites.The morphologies of the worn surfaces of the nano-composites blocks were observed using scanning elec-tron microscope(SEM,JSM-5600LV).The SEM images of the worn surfaces of EP and1.5wt.%MWNTs/EP nanocomposites are shown infigure6(a)and(b), respectively.The worn surface of pure EP shows signs of adhesion and abrasive wear(figure6(a)).The corre-sponding surface is very rough,displaying plucked and ploughed marks indicative of adhesive wear and ploughing.This phenomenon corresponds to the rela-tively poorer wear resistance of the pure EP in sliding against the steel.It can be seen that more obvious ploughed furrows appear on the worn surface of the EP block specimen.By contrast,the scuffing and adhesion on the worn surface of1.5wt.%MWNTs/EP nano-composites is considerably reduced(figure6(b)).The relatively smooth,uniform,and compact worn surfaceis Figure4.The friction coefficients of MWNTs/EP nanocomposites asa function of MWNTscontent.Figure5.Effects of MWNTs content on wear rate of MWNTs/EPnanocomposites.Figure6.SEM images of the typical worn surfaces of EP(a)andMWNTs/EP nanocomposites(b).B.Dong et al./Multi-walled carbon nanotubes/epoxy resin nanocomposites253in good agreement with the considerably increased wear resistance of the MWNTs/EP nanocomposites.There-fore,it can be deduced that the incorporation of MWNTs contributes to restrain the scuffing and adhe-sion of the EP matrix in sliding against the steel counter face.As a result,the MWNTs/EP nanocomposites show much better wear resistance than the pure EP.According to some reports[14–17],the prominent friction and wear mechanisms of MWNTs/EP nano-composites in dry sliding against a plain carbon steel counterpart may be attributed to the following two factors:firstly,the incorporation of MWNTs in EP helps to considerably increase the mechanical properties of the nanocomposites,hence the MWNTs/EP nano-composites show much better wear resistance than pure EP.Secondly,during the course of friction and wear, MWNTs dispersed uniformly in the MWNTs/EP nanocomposites may be released from the nanocom-posites and transferred to the interface between the nanocomposites and the steel counter face.Thus MWNTs may serve as spacers preventing the close touch between the steel counter face and the nanocom-posites block,which slows the wear rate and reduces the friction coefficient.What’s more,the self-lubricate properties of MWNTs also result in reduction of the wear rate and the friction coefficient.In order to make the mechanism of the tribological performance of MWNTs/EP nanocomposites more clear,further work will be done in our future study.4.ConclusionHerein MWNTs/EP nanocomposites with different contents of MWNTs were prepared successfully.The friction and wear behaviors were investigated using a friction and wear tester under dry-sliding contact condi-tions.The relative humidity of the air was about 50±10%.It was found that MWNTs significantly in-creased the wear resistance of the nanocomposites and decreased their friction coefficient.It could be seen that MWNTs/EP nanocomposites with1.5wt.%MWNTs exhibited both the smallest wear rate and the lower fric-tion coefficient.The significant improvements on the tri-bological properties of MWNTs/EP nanocomposites were attributed to the excellent mechanical properties and unmatched topological tubular structure of MWNTs. During the course of wear and friction,MWNTs which were dispersed uniformly in the nanocomposites could serve as medium,preventing the close touch of the two surfaces between the applied loading and the nanocom-posites.In conclusion,the significant improvements of tribological mechanisms of the MWNTs/EP nano-composites are attributed to the strengthening and self-lubricating functions of MWNTs. AcknowledgmentsThis work was supported by the National Natural Science Foundation of China(NNSFC,No.60471014). References[1]J.Sandler,M.S.P.Shaffer,T.Prasse,W.Bauhofer,K.Schutleand A.H.Windle,Polymer40(1999)5967.[2]L.S.Schadler,S.C.Giannaris and P.M.Ajayan,Appl.Phys.Lett.73(1998)3842.[3]C.A.Cooper,R.J.Young and M.Halsall,Composite Part A32(2000)401.[4]B.A.Rosenberg,Adv.Polym.Sci.75(1986)113.[5]D.Puglia,L.Valentini and J.M.Kenny,J.Appl.Polym.Sci.88(2003)452.[6]X.J.Xu,M.M.Thwe,C.Shearwood and K.Liao,Appl.Phys.Lett.81(2002)2833.[7]Y.Breton,G.De sarmot,J.P.Salvetat,S.Delpeux,C.Sinturel,F.Be guin and S.Bonnamy,Carbon42(2004)1027.[8]A.Allaoui,S.Bai,H.M.Cheng and J.B.Bai,Comp.Sci.Technol.62(2002)1993.[9]E.W.Wong and P.E.Sheehan,Science277(1997)1971.[10]M.M.J.Treacy,T.W.Ebbesen and J.M.Gibson,Nature381(1996)678.[11]J.P.Tu,Y.Z.Yang,L.Y.Wang,X.C.Ma and X.B.Zhang,Tri-bol.Lett.10(2001)225.[12]D.S.Lim,J.W.An and H.J.Lee,Wear252(2002)512.[13]Z.Yang,H.Xu,M.K.Li,Y.L.Shi,Y.Huang and H.L.Li,ThinSolid Films466(2004)86.[14]W.X.Chen,J.P.Tu,L.Y.Wang,H.Y.Gan,Z.D.Xu andX.B.Zhang,Carbon41(2003)215.[15]H.Cai,F.Y.Yuan and Q.J.Xue,Mater.Sci.Eng.A364(2004)94.[16]W.X.Chen,F.Li,G.Han,J.B.Xia,L.Y.Wang,J.P.Tu andZ.D.Xu,Tribol.Lett.15(2003)275.[17]W.X.Chen,J.P.Tu,Z.D.Xu,W.L.Chen,X.B.Zhang andD.H.Cheng,Mater.Lett.57(2003)1256.[18]M.Lu,Z.Wang,H.L.Li,X.Y.Guo and u,Carbon42(2004)1846.254 B.Dong et al./Multi-walled carbon nanotubes/epoxy resin nanocomposites。